This invention is related generally to luminescent materials, and specifically to antimony and manganese activated halophosphate lamp phosphors doped with rare earth elements.
Halophosphate phosphors are widely used in the fluorescent lighting industry. Such phosphors generally have the formula Ca5(PO4)3(F,Cl). The halophosphate material may contain various activator ions which impart the phosphor property. For example, a europium (Eu) activated halophosphate phosphor absorbs ultraviolet (UV) emission (i.e., exciting radiation) from the mercury plasma in a fluorescent lamp and emits blue-green visible light. However, as disclosed in U.S. Pat. No. 4,663,563, incorporated herein by reference, the Eu activated halophosphate phosphor suffers from an undesirable afterglow effect. In this effect, the phosphor luminescence continues after the excitation from the mercury plasma has been discontinued. In order to decrease the undesirable afterglow effects, the Eu activated halophosphate phosphor is doped with rare earth elements, such as Yb, Nd, Sm, Dy, Ho and Th, as disclosed in U.S. Pat. No. 4,663,563.
In contrast, an antimony (Sb) and manganese (Mn) activated halophosphate phosphor is used for white light emission in fluorescent lamps. The Sb and Mn activated halophosphate phosphor absorbs the UV emission from the mercury plasma in a fluorescent lamp and emits white light. The white light emission process is believed to comprise absorption of the 254 nm mercury emission by the Sb3+ activators, blue emission by the Sb3+ activators, energy transfer from the Sb3+ activators to the Mn2+ activators and red-orange emission by the Mn2+ activators. The combination of the blue and red-orange emission appears white to a human observer, as described on pages 114-15 of G. Blasse et al, Luminescent Materials, Springer-Verlag, 1994, incorporated herein by reference. According to the Blasse textbook, both Sb and Mn activators are believed to be located on the calcium lattice sites in the phosphor lattice. However, Blasse also notes that there is evidence that the Sb ions may also be located on the phosphorus site in the phosphor lattice. The phosphor color (i.e., xe2x80x9cwarm whitexe2x80x9d to xe2x80x9ccool white,xe2x80x9d etc.) may be adjusted by adjusting the manganese content in the phosphor, as described on page 33 of chapter 3 of K. H. Butler, Fluorescent Lamp Phosphors, Penn State University Press, 1980, the entire chapter incorporated herein by reference.
The prior art Sb and Mn activated halophosphate phosphor suffers from poor efficacy and lumen maintenance, which is a different problem than the afterglow affecting the Eu activated phosphor. Efficacy is the luminosity per unit of input electric power (measured in units of lumens/watt). Lumen maintenance is the ability of the phosphor to resist radiation damage over time. In fluorescent lamps, the prior art Sb and Mn activated halophosphate phosphor suffers from a very rapid initial decrease in the light output. This lumen depreciation in the first few minutes of lamp operation is of the same order of magnitude as the normal lumen depreciation of lamps that have been operating for 2500 hours.
It is believed that the poor efficacy and lumen maintenance are caused by UV-induced visible absorption centers, such as xe2x80x9ccolor centersxe2x80x9d and other lattice defects. Color centers are believed to be caused by lattice defects in the halophosphate lattice that trap an electron or a hole, as described on pages 79-80 of K. H. Butler, Fluorescent Lamp Phosphors, Penn State University Press, 1980, incorporated herein by reference. It has been established that the color centers are created by the 185 nm exciting radiation emitted by the mercury plasma. The color centers induce an absorption of the exciting radiation anywhere from the deep UV to the infrared region of the spectrum. Thus, these centers can degrade phosphor brightness by either absorbing the visible phosphor emission or by absorbing a part of the 254 nm mercury exciting radiation.
In the prior art, the low phosphor efficacy and lumen maintenance of the halophosphate phosphor was improved by adding cadmium to the phosphor. The addition of a few percent of cadmium to the halophosphate phosphor induced a strong absorption of the 185 nm damaging component of the mercury plasma, which reduced the intensity of this component of the plasma. Consequently, the density of the color centers created in the phosphor was reduced by adding cadmium to the phosphor. The decrease in the density of color centers in the phosphor increased the efficacy and lumen maintenance of the phosphor. However, the use of cadmium has been eliminated in phosphors manufactured in United States and Japan for public health reasons. Therefore, it is desirable to obtain a halophosphate phosphor with an improved efficacy and lumen maintenance, preferably without adding cadmium to the phosphor.
According to one aspect of the present invention, there is provided a luminescent material, comprising a host lattice excluding YVO4, at least a first defect in the host lattice which decreases an efficacy or a lumen maintenance of the luminescent material, at least one type of activator ion located in the host lattice, and a first dopant ion located in the host lattice, said first dopant counteracting an effect of said at least first defect in the host lattice.
According to another aspect of the present invention, there is provided a halophosphate phosphor comprising A D E G:Sb3+, Mn2+, R3+; where A comprises at least one of calcium, magnesium, barium, strontium or zinc; D comprises phosphorus; E comprises oxygen; G comprises at least one of fluorine, chlorine, or bromine; and R comprises at least one trivalent rare earth element.
According to another aspect of the present invention, there is provided a method of making a halophosphate phosphor comprising (a) combining at least one powder comprising at least one element selected from calcium, magnesium, barium, strontium and zinc; phosphorus; at least one halide element selected from fluorine, chlorine and bromine; oxygen; antimony; manganese and at least one trivalent rare earth element and (b) heating the at least one powder to form a solid phosphor body.